Method for manufacturing resistance spot welded joints

Abstract

To provider a spot-welding method capable of suppressing LME splitting.SOLUTION: In a method for manufacturing a resistance spot-weld joint 1, an applied pressure lowering time T2 indicating a length of a period from a time point b when a pair of electrodes X are started to be moved in a direction of separating from each other to a time point c when a pressure applied to a material to be welded by the pair of electrodes X becomes 10% of the applied pressure at the time of terminating energizing a welding current is set to 18×t2-65×t+109 or higher (t: total plate thickness of material to be welded by unit mm) by a unit msec, or an average electrode releasing speed during a period from the time point b when the pair of electrodes X are started to be moved in the direction of separating from each other to the time point c when the pressure applied to the material to be welded by the pair of electrodes X becomes 10% of the applied pressure at the time of terminating energizing the welding current is set to 30 mm / sec or lower.SELECTED DRAWING: Figure 1-2

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing resistance spot welding joints. [Background technology]

[0002] In recent years, the automotive sector has seen a demand for lighter vehicle bodies to improve fuel efficiency and reduce CO2 emissions, as well as for stronger vehicle body components to enhance crash safety. To meet these requirements, using high-strength steel plates for vehicle body components and various parts is effective. Resistance spot welding is primarily used in the assembly of automobile bodies and the installation of parts.

[0003] Resistance spot welding is a type of resistance welding in which overlapping materials to be welded are clamped between the tips of electrodes with properly shaped ends, and the welding current and pressure are concentrated on a relatively small area to heat it locally, while simultaneously applying pressure with the electrodes. In resistance spot welding, the molten and solidified portion that forms in the weld is called a nugget.

[0004] However, resistance spot welding of high-strength plated steel sheets presents a problem: LME cracking (Liquid Metal Embrittlement) occurs directly beneath the electrode or just outside the corona bond. The corona bond refers to the solid-state welded ring-shaped portion that forms around the nugget mentioned earlier.

[0005] LME cracking occurs when the heat generated during resistance spot welding melts the plating components of the plating layer, allowing alloy components to penetrate the grain boundaries of the steel sheet structure at the weld, and then tensile stress acts on the crack. LME cracking is also more pronounced under conditions where welding disturbances such as plate gap, welding angle, and clearance occur. In automobile bodies, LME cracking at welds reduces strength, and techniques are known to suppress LME cracking at welds by controlling the speed of the resistance spot welding sequence.

[0006] For example, Patent Document 1 provides a spot welding method for a member to be welded, which is composed of a plurality of steel plates with overlapping weld locations, characterized in that at least one of the plurality of steel plates has at least one overlapping surface of the weld location covered with zinc plating, the total plate thickness t (mm) of the plurality of steel plates is 1.35 mm or more, and the post-weld holding time Ht (seconds) from the end of welding current supply between the welding electrodes until the welding electrodes and the member to be welded are brought into non-contact is within the range of the following formula (A). 0.015t 2 +0.020≦Ht≦0.16t 2 -0.40t + 0.70···(A)

[0007] Furthermore, Patent Document 2 provides a method for controlling the operation of a motor that drives a gun arm for welding a workpiece, characterized in that, in a series of operations in which the gun arm moves from a state in which it is pressing on the workpiece to a state in which it is releasing this pressure and displacing the gun arm in a direction away from the workpiece, the acceleration and maximum speed of the motor in the release stage in which the gun arm releases the pressure on the workpiece are set to smaller values ​​than those in the displacement stage in which the gun arm is displaced in a direction away from the workpiece. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2017-47475 [Patent Document 2] Japanese Patent Application Publication No. 11-267852 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, the technology described in Patent Document 1 has a long pressurizing time from the end of welding current application until the electrode and the workpiece begin to separate, and a short electrode release time from the pressurizing time until the welding electrode and the workpiece are no longer in contact. In Patent Document 1, the holding time is defined as the period from the end of welding current application until the welding electrode and the workpiece are no longer in contact, but in a normal spot welding apparatus, the welding electrode is released quickly. Furthermore, Patent Document 1 does not consider the release speed of the welding electrode at all. Therefore, it is thought that the holding time described in Patent Document 1 is almost entirely the pressurizing time from the end of welding current application until the electrode and the workpiece begin to separate. If the pressurizing time is long, the welding time per point will be long when producing parts by spot welding, resulting in low productivity. In addition, in the technology described in Patent Document 2, when spot welding plated steel sheets, if the electrode release speed is fast, the residual stress in the weld area will be large, which can cause LME cracking.

[0010] The present invention has been made in view of the circumstances described above, and aims to provide a method for manufacturing resistance spot welded joints that can suppress LME cracking. [Means for solving the problem]

[0011] The gist of this invention is as follows:

[0012] (1) The welding process comprises the steps of: (1) sandwiching a workpiece, consisting of multiple overlapping steel plates with a total plate thickness of t in units of mm, between a pair of electrodes and applying pressure; (2) applying a welding current to the pair of electrodes to form a nugget; and (3) releasing the pair of electrodes by moving them toward each other, wherein at least one of the workpieces is a plated steel plate, and immediately before applying the pressure to the workpiece, one or more of the following conditions are met: clearance of 0.1 mm or more, striking angle of 0.5° or more, and plate gap of 0.1 mm or more, and the pressure reduction time, which is the length of the period from the time when the pair of electrodes begin to move toward each other until the time when the pressure applied by the pair of electrodes to the workpiece becomes 10% of the pressure at the end of the application of the welding current, in units of msec, is 18 × t 2 A method for manufacturing a resistance spot welded joint with a thickness of -65 × t + 10⁻¹ or greater. (2) The method for manufacturing a resistance spot welded joint according to (1), characterized in that the pressure reduction time is 18 × t² - 65 × t + 2¹⁴ or more in units of msec. (3) A welding material comprising the steps of: sandwiching a workpiece, which is made up of multiple overlapping steel plates and has a total plate thickness of t in units of mm, between a pair of electrodes and applying pressure; passing a welding current through the pair of electrodes to form a nugget; and releasing the pair of electrodes by moving them in a direction away from each other, wherein at least one of the workpieces is a plated steel plate, and immediately before the pressure is applied to the workpiece, one or more of the following conditions are met: clearance of 0.1 mm or more, striking angle of 0.5° or more, and plate gap of 0.1 mm or more, and the average electrode during the period from the time when the pair of electrodes begin to move in the direction away from each other until the time when the pressure applied by the pair of electrodes to the workpiece becomes 10% of the pressure at the end of the application of the welding current. Open A method for manufacturing resistance spot welded joints with a discharge velocity of 30 mm / sec or less. (4) The method for manufacturing a resistance spot welded joint according to (3), characterized in that the average electrode release speed is 10 mm / sec or less. (5) Before applying the pressing force to the workpiece to be welded, the clearance is 0.2 mm or more, the welding angle is 1° or more, and the plate gap is 0.3 mm or more, and the method for manufacturing a resistance spot welding joint according to any one of (1) to (4) is characterized in that any one or more of these conditions are satisfied. (6) The holding time, which is the length of the period from the time when the energization of the welding current ends to the time when the pair of electrodes starts to move away from each other in the direction of moving away from each other, is 100 msec or less, and the method for manufacturing a resistance spot welding joint according to any one of (1) to (5) is characterized in that this condition is satisfied.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide a method for manufacturing a resistance spot welding joint capable of suppressing LME cracking.

Brief Description of the Drawings

[0014] [Figure 1-1] It is a schematic diagram of a method for manufacturing a resistance spot welding joint. [Figure 1-2] It is a diagram showing a resistance spot welding sequence. [Figure 2] In Example 1, it is a diagram showing the change over time of the pressing force in resistance spot welding performed at various average electrode opening speeds. [Figure 3] It is a cross-sectional observation result obtained by cutting the steel plate after resistance spot welding in Example 2 in the plate thickness direction and observing the cross section. [Figure 4] It is a cross-sectional observation result obtained by cutting the steel plate after resistance spot welding in Example 3 in the plate thickness direction and observing the cross section.

Embodiments for Carrying Out the Invention

[0015] When a workpiece to be welded including a plated steel plate is resistance spot welded, if the electrodes are rapidly opened as in the conventional method, the residual stress in the welded portion increases. As a result, LME cracking occurs due to the components that have penetrated into the grain boundaries. Therefore, in the present invention, in order to improve LME cracking even with a short holding time, attention is paid to reducing the residual stress.

[0016] After the energization of the welding current is completed, the inventors of the present invention relax the impact applied to the welded portion by gently releasing the electrode, reduce the residual stress generated, suppress the penetration of the plating components of the plating layer into the grain boundaries, and also found that LME cracks directly under the electrode and immediately outside the corona bond can be suppressed. Further, as specific means for gently releasing the electrode, the inventors of the present invention also found that, for example, setting the pressure drop time to a predetermined value or more and setting the average electrode release speed to a predetermined value or less are effective.

[0017] Hereinafter, the manufacturing method of the resistance spot welding joint according to the first embodiment and the second embodiment will be described in detail. The manufacturing method of the resistance spot welding joint according to the first embodiment is characterized in that the pressure drop time is set to a predetermined value or more, and the manufacturing method of the resistance spot welding joint according to the second embodiment is characterized in that the average electrode release speed is set to a predetermined value or less. Hereinafter, first, the common points of the first embodiment and the second embodiment will be described, and then each of the first embodiment and the second embodiment will be described.

[0018] The manufacturing method of the resistance spot welding joint will be described with reference to the schematic diagram of FIG. 1-1 and the resistance spot welding sequence of FIG. 1-2.

[0019] The manufacturing method of the resistance spot welding joint includes a step S1 of sandwiching a workpiece composed of a plurality of stacked steel plates 11 with a pair of electrodes X and applying a pressing force, and then a step S2 of energizing a welding current for forming a nugget 12 to the pair of electrodes X.

[0020] [[ID=!8]] At least one of the steel plates 11 constituting the material to be welded is a plated steel plate. Plated steel plates have high corrosion resistance. Therefore, by making one or more of the materials to be welded in the resistance spot welded joint 1 plated steel plates, the corrosion resistance of the resistance spot welded joint 1 can be improved. The type of plating on the plated steel plate is not particularly limited, but can be, for example, hot-dip galvanizing or alloyed hot-dip galvanizing. The plating component is not limited to zinc-based, and the manufacturing method of the resistance spot welded joint according to the first and second embodiments can be effectively used even when a plating layer (such as aluminum) that causes LME cracking is formed on the steel plate. Examples of components that cause LME cracking in the steel plate include Zn, Sn, Cu, Cd, In, Hg, Bi, Na, Cu-Pb, Cu-Sn, Zn-Sn, Cu-Pd, Cd-Zn, Al-Sn-Cu, etc. Furthermore, while plating can cause LME cracking during spot welding, this problem is solved in the manufacturing method of resistance spot welded joints by gradually releasing electrode X.

[0021] The type of steel sheet 11 is not particularly limited, but it is preferably a high-strength steel sheet with a tensile strength of 780 MPa or more. It is also preferably a steel sheet with a carbon equivalent Ceq of 0.15% or more, as expressed by the following formula. Ceq(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 The thickness of the steel plate 11 is not particularly limited and can be appropriately selected according to the application of the resistance spot welding joint 1. The number of steel plates 11 is also not particularly limited. In Figure 1-1, there are two steel plates 11, but there may be three or more.

[0022] The manufacturing method for resistance spot welded joints includes a step S3 in which, after forming a nugget 12 and joining multiple steel plates 11, the pair of electrodes X are moved away from each other to release the pair of electrodes X. Here, "releasing the electrodes X" refers to the series of actions from the start of the movement of the electrodes X until the electrodes X and the steel plates 11 separate and the pressurizing force becomes zero. The average electrode release speed, which will be described later, refers to the rate of change over time of the distance between the tips of the pair of electrodes X when the electrodes are released. Since the steel plates 11 undergo plastic deformation under pressure, when releasing the electrodes X slowly, some time is required from the start of moving the pair of electrodes X away from each other until the pressurizing force becomes zero.

[0023] Furthermore, the spot welding according to the first and second embodiments is performed under conditions that satisfy one or more of the following conditions immediately before starting step S1, in which pressure is applied to the workpiece: clearance of 0.1 mm or more, impact angle of 0.5° or more, and plate gap of 0.1 mm or more. Clearance refers to the error when the workpiece is clamped using electrodes X. Specifically, in spot welding, which starts with both electrodes X separated from the steel plate 11 and involves bringing the electrodes X closer together, the clearance is defined as the gap between the other electrode X and the workpiece when one electrode X contacts the steel plate 11 and pressure is applied. Impact angle refers to the inclination of the mating surface of the steel plate 11, with the plane perpendicular to the line connecting the pair of electrodes X being defined as 0°. Plate gap refers to the size of the gap between the steel plates 11 in the welded area. If there are three or more steel plates 11 and two or more gaps, the plate gap means the sum of the sizes of the multiple gaps. Plate gap, welding angle, clearance, etc., are collectively called welding disturbances.

[0024] When such welding disturbances exist, residual stress in the weld tends to increase. Therefore, LME cracking is extremely likely to occur when welding disturbances are present. Consequently, satisfying the above conditions at the start of spot welding is disadvantageous from the standpoint of suppressing LME cracking. On the other hand, in spot welding at manufacturing sites, welding disturbances are sometimes unavoidable from the standpoint of improving work efficiency. Therefore, the manufacturing method of resistance spot welded joints is effective in actual working environments such as manufacturing sites. Furthermore, in the manufacturing method of resistance spot welded joints according to the first and second embodiments, the problem of LME cracking due to welding disturbances is solved by slowly releasing the electrode X.

[0025] (First Embodiment) The common components of the first and second embodiments have been described above. Next, the first embodiment will be explained with reference to the resistance spot welding sequence shown in Figure 1-2.

[0026] In the method for manufacturing a resistance spot welded joint according to the first embodiment, the pressure reduction time T2 is controlled in order to gradually release the electrodes X. The pressure reduction time T2 is the length of the period from point b, when the pair of electrodes X begin to move away from each other, to point c, when the pressure applied by the pair of electrodes X to the workpiece becomes 10% of the pressure P at point a, when the welding current is stopped. In Figure 1-2, the symbol a is attached to the point when the current value becomes 0, the symbol b is attached to the point when the decrease in pressure begins, and the symbol c is attached to the point when the pressure becomes 0.10 × P. This is because the decrease in pressure begins from the moment the pair of electrodes X begin to move away from each other.

[0027] In the first embodiment, the pressure reduction time T2 is determined according to the thickness of the material to be welded. This is because the stress due to deformation of the material to be welded during pressurization and the cooling rate of the material to be welded change according to the thickness of the material. The pressure reduction time T2 is given in units of msec as 18 × t 2The pressure must be set to -65 × t + 10⁹ or higher. Note that t is the total plate thickness of the material being welded in millimeters. This reduces residual stress in the weld and minimizes LME cracking even under welding disturbance conditions. The pressure reduction time T2 is 18 × t. 2 If the value is less than -65 × t + 10⁹ (msec), the effect of reducing LME cracking may not be sufficient, and LME cracking may occur even under small welding disturbance conditions.

[0028] Furthermore, the pressure reduction time T2 is given in units of msec, and 18 × t 2 It may be set to -65 × t + 214 or higher. In this case, the effect of reducing LME cracks can be made greater under welding disturbance conditions, and the effect of reducing LME cracks can be exerted even under conditions of large welding disturbances. The longer the pressure reduction time T2, the greater the effect of reducing LME cracks, but extending the pressure reduction time T2 may decrease productivity. Therefore, it is preferable to set an appropriate upper limit from the viewpoint of productivity.

[0029] The length of the period from point a, when the welding current is stopped, to point b, when the pair of electrodes X begin to move away from each other, is referred to as the holding time T1. Conventionally, those skilled in the art have recognized that the longer the holding time T1, the lower the residual stress in the welded joint can be. However, in the manufacturing method of the resistance spot welded joint according to the first embodiment, the holding time T1 is not particularly essential and may be 0 msec. This is because LME cracking can be suppressed by setting the pressure reduction time T2 to a predetermined value or higher. Furthermore, among LME cracks in spot welding, cracks directly outside the pressure-welded area can be suppressed by increasing the holding time T1, as described in Patent Document 1. The method for manufacturing a resistance spot-welded joint according to the first embodiment (and the method for manufacturing a resistance spot-welded joint according to the second embodiment described later) can also be effective in suppressing LME cracks directly outside the corona bond in addition to LME cracks directly outside the pressure-welded area.

[0030] (Second Embodiment) Next, the second embodiment will be explained with reference to the resistance spot welding sequence shown in Figure 1-2.

[0031] In the method for manufacturing a resistance spot welded joint according to the second embodiment, the average electrode release speed is controlled in order to release the electrode X gradually. The average electrode release speed is the average value of the electrode release speed during the period from time b, when the pair of electrodes X begin to move away from each other, to time c, when the pressure applied by the pair of electrodes X to the workpiece becomes 10% of the pressure P at time a, when the welding current is stopped. The average value of the electrode release speed can be calculated by dividing the change in the distance between the tips of the electrodes X from time b to time c by the time from time b to time c. The time from time b to time c is the pressure decrease time T2 described above.

[0032] In the method for manufacturing resistance spot welded joints according to the second embodiment, the average electrode release speed is set to 30 mm / sec or less. If the average electrode release speed is 30 mm / sec or less, residual stress in the weld can be reduced, and LME cracking can be reduced even under welding disturbance conditions.

[0033] Alternatively, the average electrode release speed may be set to 10 mm / sec or less. In this case, the effect of reducing LME cracking can be further enhanced.

[0034] In the method for manufacturing resistance spot welded joints according to the second embodiment, the holding time T1 is not particularly essential and may be 0 msec. This is because LME can be suppressed by setting the average electrode release speed to a predetermined value or higher.

[0035] The following describes more preferred embodiments of the manufacturing method for resistance spot welded joints according to the first and second embodiments. Unless otherwise specified, the embodiments described below can be applied to either the manufacturing method for resistance spot welded joints according to the first and second embodiments.

[0036] Furthermore, the manufacturing method of resistance spot welded joints in the first and second embodiments may be carried out under conditions that satisfy one or more of the following conditions immediately before applying pressure to the workpiece: a clearance of 0.2 mm or more, a striking angle of 1° or more, and a plate gap of 0.3 mm or more. Even under such more severe welding disturbance conditions, the manufacturing method of resistance spot welded joints in the first and second embodiments can demonstrate the effect of reducing LME cracking. This allows for a margin of error in the forming accuracy of the workpiece, thereby reducing the defect rate. In addition, welding equipment can be further simplified, welding preparation time can be shortened, and the burden on workers can be reduced. While there are no particular upper limits on disturbances, it is preferable that the clearance be 2.0 mm or less, the impact angle be 5° or less, and the plate gap be 2.0 mm or less.

[0037] Furthermore, the holding time T1 may be set to 100 msec or less. This shortens the cycle time and further improves manufacturing efficiency. However, when the holding time is 100 msec or less, LME cracking is particularly likely to occur in normal resistance spot welding. For this reason, in the prior art, the holding time T1 is set to more than 100 msec, and the plating is solidified before the electrode is released to reduce the risk of LME cracking. However, the manufacturing method of resistance spot welded joints in the first and second embodiments does not require a holding time T1, and even if the holding time T1 is 100 msec or less, the risk of LME cracking is low. For this reason, the manufacturing method of resistance spot welded joints in the first and second embodiments is particularly effective when it is necessary to shorten the cycle time. The value of the welding current in step S2, which involves applying the welding current to form the nugget 12, can be appropriately set so as to prevent spatter and obtain the desired nugget diameter. The value of the welding current is not particularly limited, but for example, it is 6.0 to 8.0 kA. [Examples]

[0038] The effects of one aspect of the present invention will be further specifically described by way of examples. However, the conditions in the examples are merely one set of conditions adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to this one set of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the object of the present invention.

[0039] (Example 1) A resistance spot welding test was conducted on a plurality of stacked steel plates using a spot welding device having a pair of electrodes. At this time, the behavior of the welding device was confirmed using a current-carrying pressure sensor. During welding, the welding device used stationary servo pressurization, and GA980TRIP (plate thickness: 1.4, 1.6, 2.0 mm) was used for the steel plates, two sheets of the same thickness each. In the test, the average electrode opening speed was changed to 1, 10, 20, 30, 50, 100 (mm / sec), thereby changing the pressure drop speed. An example of the relationship between each average electrode opening speed after energization and the pressure is shown in FIG. 2. Note that the cycle in FIG. 2 means 1 / 50 sec (20 msec). The welding conditions, disturbance conditions, and the presence or absence of LME cracks directly outside the corona bond after welding are shown in Table 1 below. The lower limit values of formula (1) and formula (2) described in Table 1 are values calculated by substituting the total plate thickness t into the following formula (1) and formula (2). 18×t 2 -65×t + 109 : (1) formula 18×t 2 -65×t + 214 : (2) formula <s

[0040]

Table 1

[0041] Whether or not LME cracks occurred directly outside the corona bond was determined by cutting the steel plate in the plate thickness direction so as to include the nugget and observing the cross section. As shown in Table 1, for No.2, 6, 7, 9, 10, 11, 12, 13, 17, 18, 19 where the average electrode opening speed was 30 (mm / sec) or less, there were no LME cracks. In these cases, the pressure drop time was 18×t 2-65 × t + 10⁹ (msec or more). In particular, for Nos. 10, 11, 17, 18, and 19, the pressure descent time was 18 × t 2 When performed with -65 × t + 214 or higher, no LME cracking occurred despite severe welding disturbance conditions.

[0042] (Example 2) In Example 2, a resistance spot welding test was performed. Immediately before applying pressure to the workpiece, the welding angle was 3° and the clearance was 0.3 mm. Two sheets of GA980TRIP steel (thickness: 1.6 mm) of the same thickness were used. A servo-pressurized single-phase AC welding machine was used. The welding conditions were as follows: pressure: 400 kgf, squeeze time: 600 msec (30 cycles), welding time: 400 msec (20 cycles), holding time T1: 80 msec (4 cycles, set to 1 cycle). Squeeze time is the time from when the electrode pressure command signal is issued until the welding current is started to flow.

[0043] Furthermore, two types of welding currents, 7.0 kA and 7.5 kA, were applied to each plate assembly during the test. For each welding current, two types of tests were conducted: an inventive example with a pressure descent time of 191 msec and an average electrode release speed of 10 mm / sec, and a comparative example with a pressure descent time of 58 msec and an average electrode release speed of 100 mm / sec. The cross-sectional observation results are shown in Figure 3. As can be seen from Figure 3, no LME cracks were found directly outside the corona bond in the steel plate tested for the inventive example. On the other hand, LME cracks were observed directly outside the corona bond in the steel plate tested for the comparative example. The LME cracks refer to the area enclosed by the dotted line in Figure 3.

[0044] (Example 3) In Example 3, resistance spot welding tests were performed by varying the welding current to 6.0kA, 6.5kA, 7.0kA, 7.5kA, and 8.0kA. Immediately before applying pressure to the workpiece, the hitting angle was 3° and the clearance was 0.3mm. Two sheets of GA980TRIP steel (thickness: 1.6mm) of the same thickness were used. Furthermore, a servo-pressurized single-phase AC welding machine was used. The welding conditions were as follows: pressure: 400 kgf, squeeze time: 600 msec (30 cycles), welding time: 400 msec (20 cycles), and holding time T1: 80 msec (4 cycles, set to 1 cycle).

[0045] Furthermore, at this time, tests were conducted for each welding current using the inventive example with a pressure descent time of 191 msec and an average electrode release speed of 10 mm / sec, and a comparative example with a pressure descent time of 58 msec and an average electrode release speed of 100 mm / sec. The cross-sectional observation results at this time are shown in Figure 4. As can be seen from Figure 4, no LME cracks were found directly outside the corona bond in the steel plate tested for the inventive example. On the other hand, LME cracks were observed directly outside the corona bond in the steel plate tested for the comparative example. The LME cracks refer to the area enclosed by the dotted line in Figure 4. [Industrial applicability]

[0046] According to the present invention, it is possible to provide a method for manufacturing resistance spot welded joints that can suppress LME cracking by slowing the average electrode release speed even with a short pressurizing time. Furthermore, since the pressurizing time is short, the cycle time is also short, which is expected to improve productivity in the automotive industry. Therefore, the present invention has high industrial applicability. [Explanation of Symbols]

[0047] 1. Resistance spot welding joint 11 Steel plate 12 nuggets X electrode a. The point at which the welding current is terminated. b. The point at which the pair of electrodes begin to move away from each other. c. The point at which the pressure applied by the pair of electrodes to the workpiece becomes 10% of the pressure at the end of the welding current flow. T1 retention time T2 Pressure descent time P: The pressure applied at the point when the welding current is stopped.

Claims

1. A process in which a workpiece to be welded, consisting of multiple overlapping steel plates with a total plate thickness of t in units of mm, is sandwiched between a pair of electrodes and pressure is applied, The process involves applying a welding current to the pair of electrodes to form a nugget, The steps include: moving the pair of electrodes in a direction away from each other to release the pair of electrodes; Equipped with, At least one of the materials to be welded is a plated steel sheet. Immediately before applying the aforementioned pressure to the workpiece, one or more of the following conditions are met: clearance of 0.1 mm or more, welding angle of 0.5° or more, and plate gap of 0.1 mm or more. A method for manufacturing a resistance spot welded joint, wherein the average electrode release speed during the period from the point at which the pair of electrodes begin to move away from each other in the direction described above until the point at which the pressure applied by the pair of electrodes to the workpiece becomes 10% of the pressure at the end of the current flow of the welding current is 30 mm / sec or less.

2. A method for manufacturing a resistance spot welded joint according to claim 1, characterized in that the average electrode release speed is 10 mm / sec or less.

3. A method for manufacturing a resistance spot welded joint according to claim 1 or 2, characterized in that, immediately before applying the pressing force to the workpiece, one or more of the following conditions are met: the clearance is 0.2 mm or more, the striking angle is 1° or more, and the plate gap is 0.3 mm or more.

4. A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3, characterized in that the holding time is 100 msec or less, which is the length of the period from the time when the welding current is stopped to the time when the pair of electrodes begin to move away from each other.

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